Substituted tetrazoles, and in particular tetrazolate salts substituted with nitro-, cyano-, azido- and halogens, are useful as primary explosives, as well as for other industrial applications. Conventionally, such substituted tetrazoles have been prepared through the use of cupric salts in a Sandmeyer reaction. For example, U.S. Pat. No. 4,093,623 to Gilligan et al. describes the use of Sandmeyer chemistry to prepare sodium nitrotetrazolate. In a first step, CUSO4, sodium nitrite, and 5-aminotetrazole are reacted in the presence of a strong inorganic acid to form the copper acid salt of 5-nitrotetrazole. In a second step, sodium hydroxide is used to convert the acid salt to sodium 5-nitrotetrazole.
In a manufacturing setting, this synthesis is carried out as a batch process that is plagued by certain inefficiencies. Specifically, the copper acid salt of 5-nitrotetrazole which results from the first reaction step is a precipitate that must be separated from an aqueous solution of Na2SO4 and NaNO3. In the second step, the precipitated copper acid salt is contacted with sodium hydroxide to form a precipitate of CuO in an aqueous solution of sodium 5-nitrotetrazole. The precipitated CuO must be separated from the aqueous solution to arrive at the final sodium nitrotetrazole product. Both of these precipitate-separation steps involve the use of filtration, which is unwieldy and inefficient in large-scale batch manufacturing processes, and which ultimately leads to increased manufacturing costs and lower overall yield of the final product due to inherent losses incurred during the additional process steps. Thus, there is a need for a method of manufacturing substituted tetrazoles that does not involve precipitation and filtration steps.
One solution is to remove the copper salts from the synthesis procedure altogether. In the case of sodium nitrotetrazole, this may be accomplished by reacting sodium nitrite directly with aminotetrazole and a strong inorganic acid (in the absence of CuSO4), and then neutralizing the acidic aqueous reaction product with sodium hydroxide. This procedure leads to the formation of sodium nitrotetrazole while at the same time avoiding the precipitation of any components (and filtration steps that would accompany such precipitation). It has been found, however, that the synthesis of sodium nitrotetrazole in the absence of a metal salt can lead to the accumulation of a potentially hazardous tetrazole-diazonium intermediate, which can itself lead, through deprotonation, to the formation of an equally hazardous diazotetrazole. It is theorized that the use of cuprous salts (or other metal salts) suppresses the accumulation of the tetrazole-diazonium intermediate and thereby precludes the formation of the diazotetrazole. In any event, these intermediates and unintended by-products are extremely sensitive and, when present, can spontaneously initiate a very exothermic decomposition.
Because of the sensitive nature of these intermediates, the use of a synthesis method which omits cupric salts to manufacture sodium nitrotetrazole in a commercial batch process is not feasible. There are several reasons for this. First, exothermic decomposition of the sensitive intermediates in manufacturing-scale batch equipment could result in a large explosion which could destroy the reactor equipment and cause injury to operators. Second, even if it did not result in an explosion sufficient to destroy the equipment or cause injury to operators, the exothermic decomposition would destroy all of the starting materials in the batch as well as any final product that had already been formed. As a result of these difficulties, this method has not been considered an option for the commercial manufacture of substituted tetrazoles.
Thus, there is a need in the art for a more efficient and safer process to manufacture substituted tetrazoles and substituted tetrazole salts.